I clean my pasta machine with a baby wipe between colours of clay. I wipe the underside of the rollers (underneath the machine) where the crumbles sometimes collect and I wipe the top whilst turning the handle to make sure the rollers are clean. Sometimes though I still get annoying streaks of colour across my clay and I couldn't work out where they were coming from. I decided it was time to clean my pasta machine. My husband took it apart and cleaned it for me. Great!
Not long after that a purple streak appeared on my white clay! Horror it had only just been taken apart and thoroughly cleaned! I played around trying to work out where the clay could possibly be coming from. I turned the handle to rotate the rollers backwards and there was my answer!
Pasta machine with stray purple clay on the rollers
I checked it out and if you turn the rollers backwards you get little stray bits of clay pretty much everytime. I am guessing my contamination comes when I inadvertantly rotate the rollers the wrong way a little. I guess it comes from the scrape plates at the bottom (where the crumbles collect on the outside).
Now I have incorporated rolling the rollers backwards as part of my cleaning routine and I haven't had any contamination issues since.